When foaming to a medium density in rotational molding, it is very important to use proper resins and to achieve the optimum temperature. All blowing agents decompose over a specific temperature range. In rotational molding, the resins must be fully melted and uniformly dispersed before reaching the decomposition temperature of the foaming agent. The mold temperature then increases to decompose the foaming agent, which produces the bubbles. The mold and the interior section are then rapidly cooled to help encapsulate the bubbles.
If the blowing agent decomposes before the resin is melted, the gas will escape through the spaces between the particles of unmelted resin. The resin must have good melt strength as well as good melt flow. The melt strength is the characteristic which helps to encapsulate or hold the bubbles. Amorphous or crystalline polymers do this well. The melt flow is the property which is the resistance to the vapor pressures of the gas. By improving the flow, you automatically reduce density. The melt strength cannot be sacrificed over the melt flow; otherwise, you will have large coalescing bubbles.